The present invention relates to a highly corrosion resistant Alxe2x80x94Znxe2x80x94Mgxe2x80x94Si alloy-plated steel material and to a process for its production.
Zn plating of steel surfaces for improved corrosion resistance has been widely known in the prior art, and materials with Zn platings are currently produced in mass. Znxe2x80x94Al alloy platings have even been proposed as a means of further improving corrosion resistance. Such an Znxe2x80x94Al alloy plating is proposed in Japanese Patent No. 617,971. Specifically, there is disclosed an alloy plating comprising Al at 25-75%, Si at 0.5% or more of the Al content and with the remainder consisting of substantially Zn, wherein the Znxe2x80x94Al alloy obtained exhibits excellent corrosion resistance as well as satisfactory adhesion to steel sheets and an attractive outer appearance. Such Znxe2x80x94Al alloy platings provide especially excellent corrosion resistance compared to conventional Zn platings.
It is currently the situation, however, that when Znxe2x80x94Al plated steel sheets fabricated in this manner are subjected to cutting, the exhibited corrosion resistance at the cut edges is insufficient. This occurs because, although corrosion of the steel sheet sections exposed at the cut edges is prevented by the sacrificial rusting effect of the Zn, the Zn component is lost from the Zn-segregated sections of the Znxe2x80x94Al alloy plating layer, thus lowering the corrosion resistance. Also, when the plating layer is further coated with paint or laminated with a plastic film, the corrosion product resulting from selective corrosion of Zn accumulates, creating film blisters or so-called edge creep, and thus notably reducing the product value.
As a means of improving the corrosion resistance of cut edges of painted Znxe2x80x94Al alloy platings, Japanese Patent No. 1,330,504 discloses an alloy plating containing Mg at 0.01-1.0% in a Znxe2x80x94Al alloy layer, and although a slight effect is exhibited, the technique does not provide a thorough solution to the problem of edge corrosion. A similar technique is disclosed in Japanese Examined Patent Publication HEI No. 3-21627, as a plating which comprises 3-20% Mg, 3-15% Si and the remainder Al and Zn with an Al/Zn ratio of 1-1.5, and which is characterized by having a structure with Al-rich dendritic crystals as well as Zn-rich dendritic crystals and an intermetallic compound phase comprising Mg2Si, MgZn2, SiO2 and Mg32(Al,Zn)49.
The results of experimentation by the present inventors have revealed that although plated steel sheets employing the platings disclosed in the prior art sometimes exhibit vastly improved corrosion resistance compared to Znxe2x80x94Al plated steel sheets containing no Mg or Si, the workability of the platings differs depending on the Mg and Si content, and on the proportion and the form and size of the deposited Mg2Si phase, such that considerable variation is exhibited in terms of the corrosion resistance. Particularly as concerns the size of the Mg2Si phase, the observed size also differs depending on the method of observing the structure, and especially depending on the sample embedding angle when observing the cross-sectional composition, and it was found to be important to carry out measurement of the size by a more precise method while controlling the size.
It was also found that if the content of the deposited Mg2si phase is kept at above a certain value, even outside of the range of the composition disclosed in the aforementioned prior art, there exists a range in which the corrosion resistance is vastly improved compared to conventional Znxe2x80x94Al plated steel sheets.
Another prior art example of controlling the amount of the Mg2Si phase in the plating phase is found in U.S. Pat. No. 3,026,606, which discloses a technique whereby the Mg2Si phase in the Al plating phase is controlled in a range of 4-25% and the thickness of the alloy phase produced at the interface between the plating phase and the base iron is minimized; however, the Mg2Si phase is not utilized as the means for improving corrosion resistance.
The present invention provides a highly corrosion resistant Znxe2x80x94Alxe2x80x94Mgxe2x80x94Si alloy-plated steel sheet having a controlled content of Mg and Si added to a Znxe2x80x94Al based plating and a controlled deposition amount and deposition form of the Mg2Si phase which exhibits an effect of improving corrosion resistance, as well as a process for its production.
As a result of diligent research aimed at solving the problems described above, the present inventors have completed the present invention upon finding that by adding Mg and Si in an appropriate range to Znxe2x80x94Al alloy and controlling the structure thereof, it is possible to provide an alloy plating with not only unpainted corrosion resistance but also exceptional edge creep resistance at cut edge sections after painting, which has not been achievable by the prior art.
In other words, the gist of the present invention is as follows. (
1) A Znxe2x80x94Alxe2x80x94Mgxe2x80x94Si alloy-plated steel material with excellent corrosion resistance, characterized by comprising, in terms of wt %,
Al: at least 45% and no greater than 70%
Mg: at least 3% and less than 10%
Si: at least 3% and less than 10%,
with the remainder Zn and unavoidable impurities, wherein the Al/Zn ratio is 0.89-2.75 and the plating layer contains a bulky Mg2Si phase.
(2) A Znxe2x80x94Alxe2x80x94Mgxe2x80x94Si alloy-plated steel material with excellent corrosion resistance, characterized by comprising, in terms of wt %,
Al: at least 45% and no greater than 70%
Mg: at least 1% and less than 5%
Si: at least 0.5% and less than 3%,
with the remainder Zn and unavoidable impurities, wherein the Al/Zn ratio is 0.89-2.75 and the plating layer contains a scaly Mg2Si phase.
(3) A Znxe2x80x94Alxe2x80x94Mgxe2x80x94Si alloy-plated steel material with excellent corrosion resistance according to (1) or (2) above, characterized by further comprising, as the Znxe2x80x94Alxe2x80x94Mgxe2x80x94Si alloy plating composition, one or more from among In: 0.01-1.0%, Sn: 0.1-10.0%, Ca: 0.01-0.5%, Be: 0.01-0.2%, Ti: 0.01-0.2%, Cu: 0.1-1.0%, Ni: 0.01-0.2%, Co: 0.01-0.3%, Cr: 0.01-0.2%, Mn: 0.01-0.5%, Fe: 0.01-3.0% and Sr: 0.01-0.5%.
(4) A Znxe2x80x94Alxe2x80x94Mgxe2x80x94Si alloy-plated steel material with excellent corrosion resistance, characterized in that the bulky Mg2Si phase of (1) above has a long diameter mean size of 3-50 xcexcm, the area ratio of particles with a long diameter exceeding 100 xcexcm is no more than 10% of the bulky Mg2Si phase, and the ratio of the short diameter to the long diameter is at least 0.4, as observed with a 5xc2x0 inclination polished cross-section.
(5) A Znxe2x80x94Alxe2x80x94Mgxe2x80x94Si alloy-plated steel material with excellent corrosion resistance, characterized in that the scaly Mg2Si phase of (2) above has a long diameter mean size of 3-50 xcexcm, and the ratio of the short diameter to the long diameter is less than 0.4, as observed with a 5xc2x0 inclination polished cross-section.
(6) A Znxe2x80x94Alxe2x80x94Mgxe2x80x94Si alloy-plated steel material with excellent corrosion resistance according to (1), (3) or (4) above, characterized in that the total content of the bulky and scaly Mg2Si phases in the plating layer is 10-30% as the area ratio when observed with a 5xc2x0 inclination polished cross-section, and the area ratio of bulky Mg2Si to the total Mg2Si phase is at least 1%.
(7) A Znxe2x80x94Alxe2x80x94Mgxe2x80x94Si alloy-plated steel material with excellent corrosion resistance according to (2), (3) or (5) above, characterized in that the content of the scaly Mg2Si phase in the plating layer is at least 3% as the area ratio when observed with a 5xc2x0 inclination polished cross-section.
(8) A Znxe2x80x94Alxe2x80x94Mgxe2x80x94Si alloy-plated steel material with excellent corrosion resistance according to any one of (1) to (7) above, characterized by having a preplating layer containing one or more from among Ni, Co, Zn, Sn, Fe and Cu and/or an intermetallic compound phase comprising two or more from among Ni, Co, Zn, Sn, Fe and Cu, at the interface between the plating layer and the steel material.
(9) A Znxe2x80x94Alxe2x80x94Mgxe2x80x94Si alloy-plated steel material with excellent corrosion resistance according to any one of (1) to (8) above, characterized in that the plating coverage per side is 20-130 g/m2.
(10) A process for production of a Znxe2x80x94Alxe2x80x94Mgxe2x80x94Si alloy-plated steel material with excellent corrosion resistance, which is a process for production of the Znxe2x80x94Alxe2x80x94Mgxe2x80x94Si alloy-plated steel material according to (1) to (9) above characterized by keeping the temperature of the plating bath at 500-650xc2x0 C. and controlling the cooling rate after plating to 10xc2x0 C./sec or greater.